Error Detection in Laser Beam Melting Systems by High Resolution Imaging

Laser Beam Melting as a member of Additive Manufacturing processes allows the fabrication of three-dimensional metallic parts with almost unlimited geometrical complexity and very good mechanical properties. However, its potential in areas of application such as aerospace or medicine has not yet been exploited due to the lack of process stability and quality management. For that reason samples with pre-defined process irregularities are built and the resulting errors are detected using high-resolution imaging. This paper presents an overview of typical process errors and proposes a catalog of measures to reduce process breakdowns. Based on this systematical summary a future contribution to quality assurance and process documentation is aspired.Mechanical Engineerin

Gas phase thermometry of hot turbulent jets using laser induced phosphorescence

This article is made available through the Brunel Open Access Publishing Fund. Copyright @ 2013 OSAThe temperature distributions of heated turbulent jets of air were determined using two dimensional (planar) laser induced phosphorescence. The jets were heated to specific temperature increments, ranging from 300 – 850 K and several Reynolds numbers were investigated at each temperature. The spectral ratio technique was used in conjunction with thermographic phosphors BAM and YAG:Dy, individually. Single shot and time averaged results are presented as two dimensional stacked images of turbulent jets. YAG:Dy did not produce a high enough signal for single shot measurements. The results allowed for a direct comparison between BAM and YAG:Dy, revealing that BAM is more suitable for relatively lower temperature, fast and turbulent regimes and that YAG:Dy is more suited to relatively higher temperature, steady flow situations

The development and applications of ultrafast electron nanocrystallography

We review the development of ultrafast electron nanocrystallography as a method for investigating structural dynamics for nanoscale materials and interfaces. Its sensitivity and resolution are demonstrated in the studies of surface melting of gold nanocrystals, nonequilibrium transformation of graphite into reversible diamond-like intermediates, and molecular scale charge dynamics, showing a versatility for not only determining the structures, but also the charge and energy redistribution at interfaces. A quantitative scheme for three-dimensional retrieval of atomic structures is demonstrated with few-particle (< 1000) sensitivity, establishing this nanocrystallographic method as a tool for directly visualizing dynamics within isolated nanomaterials with atomic scale spatio-temporal resolution.Comment: 33 pages, 17 figures (Review article, 2008 conference of ultrafast electron microscopy conference and ultrafast sciences

Reliable irogane alloys and niiro patination—further study of production and application to jewelry

Japanese metalworkers use a wide range of irogane alloys (shakudo, shibuichi), which are colored with a single patination solution (niiro eki). This approach allows different alloys to be combined in one piece and patinated, producing a multi-colored piece of metalwork. At present the niiro patination process is unreliable. In this study we develop a deeper understanding of the effect of patination solution ingredients on color. We have tested a synthetic niiro solution, comparing the color results with traditional niiro solution patination. Surface products have been analyzed to determine how they are influenced by both the niiro solution and cleaning procedures during patination. A large range of shibuichi and shakudo alloys have been produced to determine the full color pallette. This work also explores the use of alternative processes for the patination of irogane alloys, examining the effect of laser marking and anodizing on irogane alloys

In-situ monitoring of laser powder bed fusion applied to defect detection

Additive manufacturing technologies, particularly laser powder bed fusion (LPBF), have received much attention recently due to their numerous advantages over conventional manufacturing methods. However, the use of LPBF is still quite restricted, mainly due to two factors: its typically low productivity, which makes the technology less competitive in applications with moderate to high production volumes, and its limited reliability, particularly relevant for applications where high performance is required from the materials.The issue of low productivity is addressed in this thesis by adjusting the main LPBF process parameters. An equation for the build rate was formulated based on these parameters, determining their contributions and enabling strategies for build rate maximization. The changes in microstructure and defect populations associated with increasing productivity were determined.The reliability issue was explored by investigating defect formation, detectability and mitigation, since a major factor compromising reliability and materials’ performance is the presence of defects. Internal defects were deliberately created in LPBF-manufactured material to assess their detectability via in-situ monitoring. Two main routes of deliberate defect formation have been identified while preserving defect formation mechanisms; therefore, this thesis can be divided into two parts according to the approach employed to create defects.Defects are generated systematically if suboptimal process parameters are employed. The types, quantities, and sizes of defects in nickel-based alloy Hastelloy X resulting from varying processing conditions were thoroughly characterized. Analyzing data obtained from in-situ monitoring made it possible to distinguish virtually defect-free material from defective material.Defects are generated stochastically due to the redeposition of process by-products on the powder bed. With the aid of in-situ monitoring data, the presence of these defects can be inferred from the detection of the process by-products responsible for their formation. The comparison of data obtained in-situ with data obtained through ex-situ material characterization allowed determining how precisely detections corresponded to actual defects. The impact of these defects on the mechanical properties of Hastelloy X was assessed. A couple of in-process mitigation strategies were investigated, and their performances were evaluated. By establishing means to use LPBF process monitoring to distinguish high-quality from defective material and detect random, unavoidable defects, this thesis enables the prediction of LPBF material quality. It creates conditions necessary for the first-time-right production of defect-free material at increased build rates

Detection and classification of internal flaws in laser powder bed fusion: application of in-situ monitoring for quality control of Hastelloy X builds

Additive manufacturing technologies, in particular laser powder bed fusion (LPBF), have received much attention in recent years due to their multiple advantages over traditional manufacturing. Yet, the usage of additively manufactured products is still quite limited, mainly due to two factors: the low repeatability, which is particularly relevant for applications where high performance is required from the materials, and the typically low productivity, particularly relevant for products with a substantial production volume.The main factor that affects repeatability and compromises the performance of the materials is the presence of flaws. Hence, to assess the quality of a product and to predict its performance, it is crucial to recognize which flaws are present and ensure their detectability. Moreover, if the flaws can be detected during the manufacturing process, corrective actions can be taken. In this thesis, internal flaws were deliberately created in LPBF manufactured material to assess their detectability via in-situ monitoring. Two main routes of deliberate flaw formation have been identified while preserving flaw formation mechanisms; therefore, this thesis is split into two parts, according to the approach employed to create flaws.Flaws are generated systematically if inadequate process parameters are employed. By varying the processing conditions, different types, amounts and sizes of flaws are created. By monitoring the manufacturing process with long-exposure near-infrared imaging and applying supervised machine learning, it was possible to distinguish process conditions that generate the different flaw categories with accuracy, precision and recall of at least 96%.Flaws are created stochastically as a result of the redeposition of process by-products on the build area. It was found that substantial amounts of flaws can be provoked through this route when increasing the nominal layer thickness in the build, thus enabling the validation of the monitoring system in their detection. After applying an image analysis algorithm to all the images output from in-situ monitoring in three builds, it was possible to identify trends in the spatial distribution of spatter redeposits. Ex-situ inspection and material characterization provided cross-check for the distribution of flaws.The low productivity of LPBF makes it less competitive in applications with moderate to high production volumes. This issue is briefly addressed in this thesis. Even though one of the main approaches to increase productivity is to tune the main process parameters, dissimilar strategies were identified in the literature towards this goal. Thus, parametrization of build rates was done and applied to the processing conditions deemed to provide material with acceptable quality, based on the quantity and types of flaws present. The material manufactured in these conditions was characterized, and it was found that substantially different microstructures can be achieved within the process window, depending on the build rate

In-situ detection of redeposited spatter and its influence on the formation of internal flaws in laser powder bed fusion

The pervasive adoption of laser powder bed fusion (LPBF) as an industrial manufacturing technique relies on the improvement of its repeatability, currently limited by the stochastic formation of flaws. Considering that large flaws can form randomly and despite the optimization of process parameters, an in-situ monitoring technique suitable for detecting deviations that originate these critical flaws is paramount. The redeposition of spatters on the build area has previously been identified as one of the factors responsible for the rise of internal flaws, but so far limited are the efforts towards their detection. This study aims to detect spatter redeposits via in-situ monitoring and to couple the detections to lack of fusion. For that, long-exposure near-infrared in-situ monitoring associated with image analysis is employed to determine the exact locations and quantify the incidence of spatter redeposits across three full builds performed at varying layer thicknesses. The existence and distribution of internal flaws is verified ex-situ by means of ultrasonic inspection and metallography. The formation of internal flaws is attributed to spatter redeposits after detailed characterization of size, particle and surface morphology of spatter and identification of particles with identical characteristics on the fracture surface in the adjacencies of lack of fusion. It is found that spatters preferentially redeposit on the adjacencies of the gas outlet, but that the affected portion of the build area and the prevalence of detections is heavily dependent on the powder layer thickness employed in the manufacturing process. The monitoring system setup preferentially acquires signal from spatters redeposited on print regions, making it particularly suitable for flaw detection

Lunar meteorite regolith breccias: an in situ study of impact melt composition using LA-ICP-MS with implications for the composition of the lunar crust

Dar al Gani (DaG) 400, Meteorite Hills (MET) 01210, Pecora Escarpment (PCA) 02007, and MacAlpine Hills (MAC) 88104/88105 are lunar regolith breccia meteorites that provide sampling of the lunar surface from regions of the Moon that were not visited by the US Apollo or Soviet Luna sample return missions. They contain a heterogeneous clast population from a range of typical lunar lithologies. DaG 400, PCA 02007, and MAC 88104/88105 are primarily feldspathic in nature, and MET 01210 is composed of mare basalt material mixed with a lesser amount of feldspathic material. Here we present a compositional study of the impact melt and impact melt breccia clast population (i.e., clasts that were generated in impact cratering melting processes) within these meteorites using in situ electron microprobe and LA-ICP-MS techniques. Results show that all of the meteorites are dominated by impact lithologies that are relatively ferroan (Mg#10), and have low incompatible trace element (ITE) concentrations (i.e., typically 10 ppm Sm), High Magnesium Suite (typically >70 Mg#) or High Alkali Suite (high ITEs, Sc/Sm ratios <2) target rocks. Instead the meteorite mafic melts are more ferroan, KREEP-poor and Sc-rich, and represent mixing between feldspathic lithologies and low-Ti or very low-Ti (VLT) basalts. As PCA 02007 and MAC 88104/05 were likely sourced from the Outer-Feldspathic Highlands Terrane our findings suggest that these predominantly feldspathic regions commonly contain a VLT to low-Ti basalt contribution